1. Field of the Invention
The present invention relates to a self light-emitting device including an EL element, which is formed on a substrate, and comprises an opaque electrode (cathode), a transparent electrode (anode), and a luminescing organic material (hereinafter referred to as an organic EL material) sandwiched therebetween. Specifically, the present invention relates to an improvement of efficiency for extracting light from the EL element.
2. Description of the Related Art
In recent years, developments of EL display devices employing as an element an organic EL material are advancing. This is a current driving type self light-emitting device utilizing light emitting, which is caused by re-coupling of an electron and a hole injected into an organic thin film from the electrodes of both surfaces by applying a voltage. The light emitted is extracted as a sheet luminescent. However, the efficiency of extracting light, generated in a solid-state thin film having a large refractive index, to outside of the light-emitting element as a sheet luminescent is extremely low, normally 20% or less.
As shown in FIG. 2, among the lights outputted from a light source “A” in an “a” layer 202 having a large refractive index (n=n1) to “b” layers 201 and 203 having small refractive indexes (n=n2), the light entering at angles (θ1 and θ2) greater than the radiation angle of θ0 (provided that θ0=sin−1(n2/n1)) is entirely reflected and is waveguided in the “a” layer with a large refractive index. Thus, light that is waveguided is referred as a guided light. A part of the components of the guided light is absorbed and disappears while the rest is propagated in the “a” layer 202 and escapes to the edge surface. Therefore, only a portion of the outgoing light can be extracted as a sheet luminescent.